This invention relates to plastic ball valves for gas distribution lines, particularly for use with plastic pipe.
Valves for gas distribution lines previously have been made in many types with iron or steel valve bodies and with end connections for attachment to iron or steel pipe by threaded, flanged or welded joints. Valves used in buried service have been of rugged design to resist the hostile underground environment. While pressures in distribution service are low as compared to those encountered in transmission line valves and in valves in many other applications, distribution valves have traditionally been over-designed (from a pressure safety standpoint) to assure the dimensional stability needed for tight shutoff and resistance to pipe loads. While typical valves are operated infrequently, they require the highest order of reliability--they must be operable even in cases where years may pass between operational cycles.
The rapid growth in the use of plastic pipe, particularly polyethylene, in the last decade has had certain drawbacks on valve installations. First, costly adapters are necessary to accommodate transitions between polyethylene pipe and metal valves. Second, the metal valves and adapters compromise the plastic pipe system by leaving metal gas-containing components in the ground, with attendant questions and costs associated with corrosion protection and record keeping. Third, the effects of valve operating torque on plastic pipe require special consideration. These complications affect the life cycle cost of valve installations adversely and compromise some of the cost savings in using polyethylene pipe instead of steel pipe.
The capability of closing off a polyethylene pipe with pinch devices was recognized as a partial compensation for the increased cost of installing valves, but this expedient was considered by many as only an interim compromise. Utilities using polyethylene pipe indicated a real need for non-metallic valves compatible with the principal grades of pipe in common use.